Mucus Cocoons Protect Fish from Parasites

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Just as some people must sleep with mosquito nets to ward
off the bloodsuckers, coral reef fish apparently spin cocoons of
mucus before slumbering to keep away biting parasites, scientists
find.

Coral reef fishes such as parrotfishes and wrasses sleep soundly in
mucus cocoons that have long fascinated recreational scuba divers
and are often a main attraction on night dives. Surprisingly,
until now, no experimental studies had examined their function.
[Image
of parrotfish with cocoon]

Past observational studies have suggested these gooey
blankets, which are secreted from the fishes' mouths, somewhat
protect fish from nocturnal predators such as moray eels.
However, researchers also found many cocooned fish were still
eaten.

Fishy mosquito nets

To find out why these cocoons are created, scientists
concentrated on parasitic crustaceans known as gnathiid isopods
(tiny shrimp-like creatures), which feed on the blood of fish.
During the day, parrotfish seek out cleaner fish to remove these "marine
mosquitoes," explained researcher Alexandra Grutter, a marine
biologist at the University of Queensland in Australia.

For the study, researchers focused on
the bullethead parrotfish (Chlorurus sordidus). Each
parrotfish has to be caught separately by a scuba diver using a
large 6-foot-by-45-foot (1.8 meter by 13.7 meter) net; swimming
underwater with a rolled-up net takes stamina, Grutter said.
Also, "parrotfish are extremely slippery so getting them from a
hand net to a catch bag is very tricky," Grutter added.
"Parrotfish also have a mouth shaped like a beak, hence their
name, so you want to avoid sticking your finger in their mouth
when putting them in a bag."

Back at the lab, the researchers compared fish coated with
mucus with ones the researchers gently pushed out of their
cocoons without disturbing their snooze. The slumbering
fish were then exposed to the parasites for 4.5 hours.

The researchers estimated crafting these cocoons takes up
2.5 percent of the fish's daily energy.

"The amount of effort that goes into building these
cocoons, which requires fish to have developed very large glands
about the size of [a] quarter to produce the cocoons, is
extraordinary," Grutter said. "Parasites must exert an enormous
pressure on these fish in order for the fish to have evolved such
a specific way of avoiding the parasites."

The parasites might not simply only wound the fish. "These
parasites have been implicated in transmitting a blood parasite,
which resembles malaria," Grutter explained.

It remains uncertain exactly how the cocoons protect the
fish. "Are they a physical and/or chemical barrier, or do the
cocoons prevent the parasites from detecting fish by blocking
fish
odor?" Grutter asked. "What other organisms are deterred by
the cocoons? Research in the wild would be interesting, but this
is inherently dangerous due to the dangers of working underwater
in the dark and other nocturnal predators of humans, such as
sharks."

It is also an open question as to whether the fish can
recoup part of their investment by eating the cocoons.

"I have observed on occasion a fish at dawn with what
appeared to be mucus stuffed in its mouth," Grutter told
LiveScience. "I have observed another fish that produces cocoons,
the cleaner fish Labroides dimidiatus, pecking away at
its old cocoons in the morning."

The scientists detailed their findings Nov. 17 in the
journal Biology Letters.